<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>https://codepen.io/Chokcoco/pen/wvygVLd</title>
</head>
<link rel="stylesheet" href="../common.css">
<style>
    .parent {
        width: 400px;
        height: 200px;
        margin: auto;
        filter: drop-shadow(0 0 0.2px #03a9f4) drop-shadow(0 0 0.2px #03a9f4) drop-shadow(0 0 0.2px #03a9f4) drop-shadow(0 0 0.2px #03a9f4) drop-shadow(0 0 0.2px #03a9f4) drop-shadow(0 0 0.2px #03a9f4) drop-shadow(0 0 0.2px #03a9f4) drop-shadow(0 0 0.2px #03a9f4) drop-shadow(0 0 0.2px #03a9f4) drop-shadow(0 0 4px #03a9f4) drop-shadow(0 0 4px #03a9f4);
    }

    .child {
        height: 200px;
        -webkit-mask: linear-gradient(135deg, transparent, transparent 20px, #000 20px);
        mask: linear-gradient(135deg, transparent, transparent 20px, #000 20px);
        background: rgba(0, 0, 0, 0.8);
    }
</style>

<body>
    <div class="parent">
        <div class="child"></div>
    </div>
</body>

</html>